In producing and cleaning electronic parts, media specially prepared for these purposes have conventionally been used, such as, e.g., sulfuric acid, hydrofluoric acid, hydrogen peroxide and hydrochloric acid. These cleaning media will continue to be suitably used depending on the intended application. However, because these cleaning media are obtained by specially purifying corresponding products produced through chemical processes, the purification operations are complicated. This is because the purification involves the step of removing metallic ingredients which have contaminated the chemical products, for example, from the catalysts used for producing the same. As a result, the purified products are expensive. In addition, even if the purification operations are carefully conducted, the thus purified products cannot always provide the reduction in allowable impurity amounts needed with the advancement in electronic devices. New substitute techniques are hence desired.
One of these substitute techniques is the use of ozonized water. In particular, highly ozonized water produced by electrolysis is known to be exceedingly effective, e.g., in cleaning electronic devices. However, because the use of ozonized water alone is insufficient in some cases, there is a growing need for a treatment liquid which has one or more functions not possessed by ozonized water, e.g., an oxidizing function and a reducing function, and which contains no metallic elements at all.
Among such treatment liquids is a so-called acidic water or ultra-acidic water. The acidic water generally has a pH of 3 or lower and an oxidation-reduction potential (ORP) of 1.2 V or higher and hence has an oxidizing capability. Consequently, the acidic water has the effect of, for example, decomposing organic substances or dissolving metallic deposits therein to remove these impurities, and is being used, although in a small amount, for the cleaning of electronic devices, etc.
Simultaneous with the production of the acidic water in an electrolytic cell, alkaline water having a pH of 10 or higher and an ORP of 0 V or lower is produced as a by-product in the cathode chamber of the electrolytic cell. Investigations on the use of this alkaline water for cleaning, etc., have been initiated.
In the electrolytic production of modified acidic water and alkaline water (cleaning waters), a two-chamber type electrolytic cell is generally used which has been partitioned into an anode chamber and a cathode chamber with an ion-exchange membrane serving as a diaphragm. For conducting electrolysis using this electrolytic cell, an appropriate supporting electrolyte is added to the electrolyte liquid in order to impart ionic conductivity thereto. However, in most cases, the cleaning water thus produced contains the supporting electrolyte remaining therein or is contaminated with metallic ions and particles. These contaminants are attributable to dissolution of the material constituting the inner wall of the electrolytic cell main body in the electrolyte liquid. If such contaminated cleaning water is used for cleaning electronic devices such as semiconductors and liquid crystals, metallic ions and other contaminants contained in the cleaning water adhere to the semiconductor surfaces and can cause insulation failures.
Consequently, for further improving the purity of the cleaning water, a proposal has been made to use ultrapure water as an electrolyte without using a cleaned supporting electrolyte which is purified by removing contaminants before supplying it to the cell. The cleaning water thus produced by electrolysis has a high purity satisfactory even for semiconductor cleaning.
In the above electrolytic operation, a DC power supply employing a rectifying element such as a selenium or silicon rectifier is used to supply power to the electrolytic cell for water electrolysis. In this electrolytic operation, cleaning-water production is not continuous over the course of 24 hours, and power is usually supplied for several hours per day at the most. Thus, the electrolytic cell is mostly in a power supply cutoff state. However, the DC power supply has no electromotive force in a power supply cutoff state, as in the above suspension of operation or during service interruption. Furthermore, the anode and cathode of the power supply are electrically connected to each other through the electrolytic cell for water electrolysis when a liquid is present in the cell. Consequently, in such circumstances, the electrolytic cell functions as a battery such that a reverse current flows through the electrolytic cell. This results in the following adverse effects. Namely, the reverse current not only causes an electrode material covered with an electrode to dissolve out to thereby deactivate the electrode, but also, the dissolved electrode material is eluted into the acidic water or alkaline water thus produced to thereby contaminate the same.
In an electrolytic cell for electrolytic ozone gas generation which is used for water electrolysis for generating an oxygen-containing ozone gas in the anode chamber, a DC power supply circuit capable of always supplying power to the anode chamber side even in a power supply cutoff state is generally used. This prevents the anode material from being deactivated in a current cutoff state, and therefore prevents the anode material from being reduced by a reverse current generated in a current cutoff state. As a result, restarting the power supply does not result in the same current efficiency for the ozone generation as before.
Besides the use of the above-described DC power supply circuit, an effective means for preventing the generation of a reverse current in a power supply cutoff state may be to interpose, between the DC power supply and the electrolytic cell for water electrolysis, a DC circuit breaker which opens when the power supply is turned off. However, the use of a circuit breaker is disadvantageous in that when a large current is supplied, the contact point of the breaker deteriorates when the current is turned off. Furthermore, because a reverse current flows in an amount corresponding to the internal impedance of the breaker, this results in deterioration of the electrode in a power supply cutoff state and contamination of the electrolyte liquid.